The semiconductor industry has been seeking more cost effective solutions for manufacturing BiCMOS devices for mass applications of radio frequency (RF)/analog and wireless/fiber-based telecommunications for decades. Si/SiGe BiCMOS technology is widely used and has been quite successful. However, as complementary metal oxide semiconductor (CMOS) adopts thin silicon-on-insulator (SOI) substrates for lower power and higher speed (due to device scaling), the thick subcollector of conventional bipolar junction transistors (BJTs) becomes incompatible with the integration of high-performance SOI CMOS devices.
In order to facilitate integration with SOI CMOS, lateral SOI BJTs have been proposed and studied. See, for example, S. Parke, et al. “A versatile, SOI CMOS technology with complementary lateral BJT's”, IEDM, 1992, Technical Digest, 13–16 Dec. 1992, page(s) 453–456; V. M. C. Chen, “A low thermal budget, fully self-aligned lateral BJT on thin film SOI substrate for lower power BiCMOS applications”, VLSI Technology, 1995. Digest of Technical Papers. 1995 Symposium on VLSI Technology, 6–8 Jun. 1995, page(s) 133–134; T. Shino, et al. “A 31 GHz fmax lateral BJT on SOI using self-aligned external base formation technology”, Electron Devices Meeting, 1998. IEDM '98 Technical Digest, International, 6–9 Dec. 1998, page(s) 953–956; T. Yamada, et al. “A novel high-performance lateral BJT on SOI with metal-backed single-silicon external base for low-power/low-cost RF applications”, Bipolar/BiCMOS Circuits and Technology Meeting, 1999. Proceedings of the 1999, 1999, page(s) 129–132; and T. Shino, et al. “Analysis on High-Frequency Characteristics of SOI Lateral BJTs with Self-Aligned External Base for 2-GHz RF Applications”, IEEE, TED, vol. 49, No. 3, pp. 414, 2002.
Even though lateral SOI BJT devices are easier to integrate with SOI CMOS, the performance of such devices is quite limited. This is because the base width in the lateral SOI BJTs is determined by lithography. Hence, it cannot be scaled down (less than 30 nm) readily without more advanced and more expensive lithography technologies such as e-beam lithography.
Another type of SOI BJT, which is a vertical SOI SiGe bipolar device, has also been proposed and demonstrated to offer higher base-collector breakdown voltage, higher early voltage and better BVCEO-fT tradeoff. This type of SOI BJT is described, for example, in J. Cai, et al., “Vertical SiGe-Base Bipolar Transistors on CMOS-Compatible SOI Substrate”, 2003 IEEE Bipolar/BiCMOS Circuits and Technology Meeting. This SOI BJT device uses a fully depleted SOI layer as the collector at zero substrate bias. The application of a substrate bias to this SOI BJT device allows for significant improvement in overall device performance by reducing collector space-charge region transit time and collector resistance through the formation of an accumulation layer.
A problem with the SOI BJT device described above is that the buried oxide (BOX) layer in high performance CMOS SOI substrates is typically 100–200 nm thick. As a result, the substrate bias needed for significant performance improvement is unacceptably large (greater than about 20 V). In order for these devices to be practical for SOI BiCMOS applications, the substrate bias must be held at or below the voltage applied to the CMOS, typically less than 3 V.
In view of the above, there is a need for providing a SOI BJT structure that overcomes the drawbacks mentioned in the prior art SOI BJTs.